Characterization of the fibre-matrix interfacial structure in carbon fibre-reinforced polycarbosilane-derived SiC matrix composites using STEM/EELS

This paper presents a characterization study of the microstructural evoluti on of various carbon fibre-reinforced polycarbosilane (PCS)-derived SiC mat rix composites during high temperature heat treatment. Both surface-treated and untreated carbon fibre reinforcements were investigated. The STEM/EELS technique was found to be a particularly useful characterization tool. The results of quantitative EELS linescans have been interpreted in terms of t he migration of gaseous SiO and CO, produced by the reaction between the sm all amount of SiO2 and excess carbon within the PCS-derived SiC matrix, fro m the central matrix region towards the fibre-matrix interfaces, Generally, the migration of gaseous SiO and CO results in an enrichment of SiO2 at th e region adjacent to the fibre-matrix interface. However, differing final c omposite microstructures are formed depending on the strength of the fibre- matrix bonding, In the case of strong fibre-matrix interfacial bending wher e few escape channels are present, a distinct Si-C-O layer was identified w ithin the matrix adjacent to the fibre-matrix interface: both crystalline b eta-SiC and the segregated Si-O-C phase coexist in this microstructure up t o at least 1450 degrees C, In the case of weak fibre-matrix bonding this ox ygen segregated interfacial layer is eventually removed at high enough temp eratures. The final interfacial microstructure has important consequences f or the mechanical properties of the composite material.